Validation of SO₂ retrievals from the Ozone Monitoring Instrument over NE China

The Dutch-Finnish Ozone Monitoring Instrument (OMI) launched on the NASA Aura satellite in July 2004 offers unprecedented spatial resolution, coupled with contiguous daily global coverage, for space-based UV measurements of sulfur dioxide (SO₂). We present a first validation of the OMI SO₂ data with in situ aircraft measurements in NE China in April 2005. The study demonstrates that OMI can distinguish between background SO₂ conditions and heavy pollution on a daily basis. The noise (expressed as the standard deviation, σ) is ∼1.5 DU (Dobson units; 1 DU = 2.69 · 10¹⁶ molecules/cm²) for instantaneous field of view boundary layer (PBL) SO₂ data. Temporal and spatial averaging can reduce the noise to σ ∼0.3 DU over a remote region of the South Pacific; the long-term average over this remote location was within 0.1 DU of zero. Under polluted conditions collection 2 OMI data are higher than aircraft measurements by a factor of two. Improved calibrations of the radiance and irradiance data (collection 3) result in better agreement with aircraft measurements on polluted days. The air mass-corrected collection 3 data still show positive bias and sensitivity to UV absorbing aerosols. The difference between the in situ data and the OMI SO₂ measurements within 30 km of the aircraft profiles was about 1 DU, equivalent to ∼5 ppb from 0 to 3000 m attitude. Quantifying the SO₂ and aerosol profiles and spectral dependence of aerosol absorption between 310 and 330 nm are critical for an accurate estimate of SO₂ from satellite UV measurements.